1. Field of the Invention
The present invention is directed to increasing the storage stability of blocked polyisocyanates by adding a stabilizing amount of a compound having monofunctional reactivity toward isocyanates.
2. Description of the Prior Art
Coating compositions based on a blocked polyisocyanate component and a component containing isocyanate-reactive hydrogens are known. The purpose of the blocking agent is to prevent the polyisocyanate from reacting with the isocyanate-reactive component at ambient temperature conditions and thus allows the two components to be mixed and stored prior to their actual use. When the composition is baked at an elevated temperature, the blocking agent is released and the reaction of the two components commences. When using common blocking agents such as .epsilon.-caprolactam, unblocking temperatures in excess of 170.degree. C. are needed to provide acceptable rates of unblocking.
The high temperatures needed for unblocking are unfavorable for two reasons. First, the high temperatures can cause yellowing of the reacted polyurethane compositions. Second, the energy requirements are much higher when compared to competitive systems based on aminoplast resins and polyhydroxyl compounds which can be baked at temperatures of as low as about 125.degree. C. using acid catalysis. Since the unblocking temperatures of the conventional polyisocyanate-based systems are much higher, the manufacturing facilities designed for the competitive aminoplast systems cannot accommodate the conventional blocked polyisocyanate systems. Accordingly, even though the compositions based on blocked polyisocyanates and compounds containing isocyanate-reactive hydrogens yield products with a superior combination of hardness and elasticity when compared to the competitive systems, the need exists for a polyisocyanate-based system which is stable under ambient conditions and wherein the blocked polyisocyanate component may be reacted at lower temperatures within an acceptable period of time.
While it is known from U.S. Pat. Nos. 2,801,990; 3,779,794; 4,007,215; 4,087,392; 4,101,530; 4,132,843 and 4,332,965; British Pat. Nos. 1,442,024 and 1,523,103; German Offenlegungsschrift No. 2,623,081 and German Auslegeschrift No. 2,639,491 that polyisocyanates blocked with C--H acidic compounds such as malonic acid esters and acetoacetic acid esters can be reacted at lower temperatures, it has been found that when combined with suitable co-reactants, these systems do not remain stable. When these systems are stored, the viscosity gradually increases until the systems gel. The higher the storage temperature, the faster gelation occurs.
The addition of stabilizers having monofunctional reactivity towards isocyanate groups to improve the storage stability of these systems forms the basis of U.S. application Ser. No. 498,225, filed May 26, 1983, now U.S. Pat. No. 4,437,593. It was also proposed to improve the storage stability of these systems by separately storing the two components. However, it was unexpectedly found that polyisocyanates blocked with C--H acidic compounds do not remain stable even in the absence of a suitable co-reactant.
Accordingly, it is an object of the present invention to improve the storage stability of polyisocyanates blocked with malonate-based blocking agents. It is an additional object to provide polyisocyanates blocked with malonate-based blocking agents which when mixed with suitable co-reactants can be reacted at lower temperatures than conventional blocked polyisocyanate systems and result in polyurethanes possessing properties which are superior to the competitive, low temperature systems.
These and other objects may be achieved by proceeding in accordance with the present invention as described below.